Plasma is used in etching, deposition, and sputtering processes for manufacturing semiconductors and liquid crystal displays (LCDs). More specifically, plasma is formed in a vacuum chamber (i.e., a plasma chamber) using electricity supplied in the form of a high-frequency voltage or current, and the above-mentioned processes are performed using the physical or chemical characteristics of ions of the plasma.
To satisfy certain technical demands related to integration level and performance, highly advanced techniques have been applied to the processes, resulting in changes to process conditions. However, the repeatability of the processes or product performance can be largely affected by even a small variation in process condition, and therefore, it may be necessary to monitor variations of process conditions.
However, it can be difficult to evaluate the repeatability of a process by monitoring process conditions since various process conditions can affect the repeatability of the process. For example, process conditions, such as gas flow, unintentional fluctuation, chamber memory effect, arching, and plasma instability, can affect the repeatability of the process and product performance. It can be difficult to precisely measure all of such various process conditions. In addition, it can also be difficult to predict changes of process outputs from the measurement results.
The state of a process in which plasma is used can be monitored based on the fact that the state of the process is sensitive to the state of the plasma. For example, an etch stop point can be determined by measuring variations in optical characteristics of plasma. More specifically, when a lower layer disposed beneath an etch target layer is exposed while etching the etch target layer, gas ratios and pressure of a chamber vary, and thus the intensity of light emitted from plasma can vary at a specific wavelength. The etch stop point can be determined by monitoring variations of the intensity in the light emitted from the plasma. However, this method can be used to determine only the etch stop point since the method uses light having a specific wavelength. That is, the repeatability of a process may not be properly evaluated by the method.